Intervertebral distractors are devices adapted to space two adjacent vertebrae. In particular, the distractors according to the present invention are prostheses for steady implantation in the space set between the spinous processes of two adjacent vertebrae, in order to maintain an intervertebral distraction adapted to limit the loads transmitted between the vertebrae in case of degenerative diseases of the intervertebral disc, thus limiting the associated painful effects.
With respect to other vertebral prostheses, interspinous distractors can be easily implanted, in view of the relative simplicity according to which the spinous processes of two adjacent vertebrae can be slightly spread apart. For the same reasons, such distractors do not jeopardize the local mobility of the spine when bending, and reduce hyperextension. Notwithstanding such advantages, known stabilization problems exist. In other words, an interspinous distractor has to be kept in position, in particular it has to be constrained with respect to movements, thus affecting its functionality or causing it to exit from the interspinous gap, with movements in a plane orthogonal to the spine.
In particular, WO2006102269 describes interspinous distractors for keeping the implant within the interspinous gap, comprising a central portion adapted to be contained in the interspinous gap providing an interspinous support adapted to maintain a desired distraction, and stop members consisting of two ends portions, one of which is fixed while the other is moveable, the latter comprising elements movable according to two extreme positions. The fixed end has a profile and a size such that it can be approached laterally against the spinous processes of the two adjacent vertebrae. Adjusting means are provided for the moveable end, on which the surgeon acts once the device is implanted, thus forming a bilateral limitation to sliding for the distractor according to its own longitudinal axis. The interspinous and the supraspinous ligament assist settling of the implant between the spinous processes, in particular with respect to movements in a plane orthogonal to the spine, thus ensuring a local mobility of the spine.
However, such device cannot be implanted percutaneously, in view of an excessive size of the fixed portion of the stop members, which influences the overall size of the distractor, and also because the means for adjusting the movable part of the stop members cannot be operated percutaneously.
A further device is also known, called Synthes®, comprising a central body with a substantially cylindrical shape and two couples of movable and reversible stop members that project from the central body, such that the radial size of the device, in a closed position, does not exceed the substantially cylindrical body. Once the body has been endoscopically placed between the spinous processes, the surgeon acts always percutaneously on position adjusting elements of such mobile stop members, causing them to partially exit from the body through suitable slits, the stop members being conformed to hook the spinous processes. Even in this case, the interspinous ligament assists placement of the prosthesis between the spinous processes. The mobile stop members, however, being housed in the cylindrical body, have to rotate in order to protrude from the respective slit. This entails a high internal mechanical complexity. Furthermore, the stop members have a portion remaining in the cylindrical body that is large enough to support the part protruding from the body, in order to assure enough blocking force to the distractor. The presence of the stop members and of the relative mechanisms in the cylindrical body does not allow having distractors under a certain size.
Examples of interspinous distractors, particularly devised for cervical vertebrae, are disclosed in US20060271049A1, US2007010813A1 and in US US2008108990A1. These distractors have a body from which a wing extends, which wing can move from a closed position, not exceeding the height of the body, to a spread apart position protruding laterally with respect to the body, and can then keep the body settled between two spinous processes, in order to carry out distraction and at the same time blocking an accidental movement of the body.